Two-part medical tracking marker

ABSTRACT

The invention relates to a medical tracking marker ( 12 ) for localizing or tracking an object ( 10 ) by means of a medical tracking system ( 30 ), said marker comprising a first part ( 1 ) which is to be fixed on the object ( 10 ), and a second part ( 2 ) which is to be connected to the first part ( 1 ), wherein the first and second parts ( 1, 2 ) comprise interconnecting elements ( 4, 6 ) which are adapted to provide a positionally determined connection between the first part ( 1 ) and the second part ( 2 ) while the first part is located under a surgical drape ( 20 ) and the second part is located above said surgical drape ( 20 ), with the drape ( 20 ) placed between the two parts ( 1, 2 ). The invention also relates to a method of referencing an object ( 10 ) using a medical navigation system, wherein a medical tracking marker such as has been described above is localized or tracked by means of a medical tracking system ( 30 ), and wherein the object is registered by means of the first part ( 1 ) of the marker ( 12 ) in a preliminary referencing step, in particular for imaging purposes, which is later adapted to form a final referencing step for navigation using the second part ( 2 ) which has been placed on the first part in a positionally defined, pre-determined manner.

This application is a national phase of International Application No.

PCT/EP2009/062685 filed Sep. 30, 2009 and published in the Englishlanguage.

The present invention relates to a medical tracking marker which is usedwhen an object is to be localized or tracked by means of a medicaltracking system, preferably an optical medical tracking system. Suchmedical tracking markers are known in general for example from DE 196 39615 A1 and are used in conjunction with a medical navigation system. Theabove-referenced document discloses a multi-part marker systemcomprising one part which is to be fixed on the patient and twodifferent marker parts which, alternatively, may be attached to saidpart for different purposes, for example one fiducial marker for imagingpurposes and a second one for tracking purposes. Other known markersystems involve invasively fixing individual markers or a head ringbearing such markers, wherein fixing includes penetrating the bone. Suchsystems are particularly problematic in pediatrics owing to their sizeand invasiveness. Another problem to be dealt with concerns the need todrape the patient in order to provide a sterile environment, since themarkers must somehow be arranged above the drapes for tracking purposesbut fixed below the drapes. In accordance with the prior art, either thedrapes are arranged around the markers or the markers penetrate thedrapes, both of which methods compromise sterility.

In general, it is the function of a marker to be detected by a markerdetection device (for example, a camera or an ultrasound receiver), suchthat its spatial position (i.e. its spatial location and/or alignment)can be ascertained. Such markers can be active markers. An active markeremits for example electromagnetic radiation and/or waves, wherein saidradiation can be in the infrared, visible and/or ultraviolet spectralrange. The marker can also however be passive, i.e. can for examplereflect electromagnetic radiation from the infrared, visible and/orultraviolet spectral range. To this end, the marker can be provided witha surface which has corresponding reflective properties. It is alsopossible for a marker to reflect and/or emit electromagnetic radiationand/or waves in the radio frequency range or at ultrasound wavelengths.A marker preferably has a spherical and/or spheroid shape and maytherefore be referred to as a marker sphere; markers can also, however,exhibit a cornered—for example, cubic—shape or a disk shape.

It is the object of the present invention to provide a medical trackingmarker and a method of referencing an object using a medical navigationsystem by using such a marker, which provide a wide range of applicationand preserve sterility. This object is achieved by a tracking marker inaccordance with claim 1 and a method of referencing an object using amedical navigation system in accordance with claim 13. The sub-claimsdefine advantageous embodiments of the invention.

The tracking marker of the present invention comprises a first partwhich is to be fixed on the object, and a second part which is to beconnected to said first part. The first and second parts compriseinterconnecting elements which are adapted to provide a positionallydetermined connection between the first part and the second part whilethe first part is located under a surgical drape and the second part islocated above said surgical drape, with the drape placed between the twoparts.

In other words, the present invention adapts the marker configuration byaccepting that draping must take place and by taking the presence of adrape into account in designing the marker. In particular, providing apositionally determined connection between the parts which are“separated” by the drape enables accurate tracking and localizing, whilea large degree of freedom is provided in the design with respect tofixing the first part on the object, for example on the patient's skin.In this way, the invention provides an accurate and robust trackingmarker technology and excellent sterility conditions.

In one embodiment of the invention, the marker parts are not in directcontact when connected. In particular, their interconnecting elementsare not in direct contact with each other. Said interconnecting elementsmay, when connected, be spaced apart by at least a distancecorresponding to the thickness of the surgical drape, wherein theconnection bridges said distance. The marker according to the inventionmay be designed in such a way that the interconnecting elements do notpenetrate the drape when connected. The marker parts, when connected,may clamp the drape without penetrating it. One way of establishing sucha connection is to use forces which act over a distance, in particularmagnetic or electrostatic forces (or fields). In this case, the partsmay be provided with two different magnetic or static domains in theconnecting area, in particular on each surface involved in theconnection, thereby securing a predetermined alignment of the partswhich are connected without contacting each other.

On the other hand, a positive connection between the interconnectingelements may be chosen in order to connect the two marker parts, whereinthe drape is accommodated between said elements which in particular forma bayonet catch or a quarter-turn fastener.

Another way of connecting the parts involves a frictional engagementbetween the interconnecting elements, wherein the drape is preferablyaccommodated between said elements and thus forms a part of thefrictional connection.

In another embodiment of the medical tracking marker in accordance withthe invention, the first part of said marker is fixed on the objectnon-invasively, i.e. said first part comprises a non-invasive objectconnection means which for example adhesively bonds the marker part ontothe object or fixes it there by means of adhesive tape. Suchnon-invasive fixing techniques are particularly suitable for pediatricsor pediatric cranial procedures in which conventional reference systemsare ill-suited due to the size of a child's head and the limitedrigidity/thickness of the cranial bone. One of the first and secondparts, in particular the second part, or both parts may be provided inaccordance with the invention with a reflective surface which can bedetected by the tracking system, such that at least the second part, butin some instances (where useful) also the first part, may be used fordirect tracking purposes.

The method of the present invention is a method of referencing an objectusing a medical navigation system, wherein a medical tracking markersuch as has been described above in various embodiments is localized ortracked by means of a medical tracking system, preferably an optionalmedical tracking system, and wherein the object is referenced by meansof the first part of the marker in a preliminary referencing step, inparticular for imaging purposes, which is later adapted to form a finalreferencing step for navigation using the second part which has beenplaced on the first part in a positionally defined, predeterminedmanner.

Thus, referencing and registration are advantageously combined in themethod in accordance with the invention.

A navigation system and/or surgical navigation system is understood tomean a system consisting of: at least one marker device; a transmitterwhich emits electromagnetic waves and/or radiation and/or ultrasoundwaves; a receiver which receives electromagnetic waves and/or radiationand/or ultrasound waves; and an electronic data processing device whichis connected to the receiver and/or the transmitter, wherein the dataprocessing device (for example, a computer) comprises in particular aprocessor (CPU), a working memory, advantageously an indicating device(for example, a visual indicating device such as a monitor and/or anaudio indicating device such as a loudspeaker) and advantageously apermanent data memory, wherein the data processing device processesnavigation data forwarded to it by the receiver and can advantageouslyoutput guidance information to a user via the indicating device. Thenavigation data can be stored in the permanent data memory and forexample compared with data which has been stored in said memorybeforehand.

With respect to the term “registration”, the n-dimensional image of abody is registered when the spatial location of each point of a realobject within a space, for example a body part in an operating theatre,is assigned an image data point of an image (CT, MR, . . . ) stored in anavigation system. Determining the position is called “referencing” ifit implies informing a navigation system of said position in a referencesystem of the navigation system.

In one embodiment of the method in accordance with the invention, theadapting step compensates for and takes into account the shift in theposition of the marker which occurs when the second part is placed onthe first part, and in particular takes into account one or more of thefollowing:

-   -   the thickness of the drape;    -   a specific shape of the second part;    -   characteristics of or inaccuracies in the system connecting the        parts;    -   a shift in the skin;    -   a movement of the tracking system.

One way of taking such a shift in position into account is to ignoremarkers which are found to have shifted during the medical navigation.

The invention also relates to a computer program which, when it isrunning on a computer or is loaded onto a computer, causes the computerto perform a method as described above, and to a computer programstorage medium which comprises such a computer program.

With respect to the above, the method in accordance with the inventionis in particular a data processing method. The data processing method ispreferably performed using technical means, in particular a computer.The computer in particular comprises a processor and a memory in orderto process the data, in particular electronically. The calculating stepsdescribed are in particular performed by a computer. Steps of definingfor example regions or values are in particular steps of determiningdata within the framework of the technical data processing method, inparticular within the framework of a program. Altering steps inparticular represent altering the data by means of the computer.Ascertaining steps in particular include retrieving values which areprovided at an interface of the computer and have been generated bytechnical means, such as for example a scanning device. These values arein particular converted by the interface into data which can beprocessed by the computer.

Computer program elements of the invention may be embodied in hardwareand/or software (including firmware, resident software, micro-code,etc.). The computer program elements of the invention may take the formof a computer program product which may be embodied by a computer-usableor computer-readable storage medium comprising computer-usable orcomputer-readable program instructions, “code” or a “computer program”embodied in said medium for use by or in connection with the instructionexecuting system. Such a system can be a computer; a computer can be adata processing device comprising means for executing the computerprogram elements and/or the program in accordance with the invention.Within the context of this application, a computer-usable orcomputer-readable medium may be any medium which can contain, store,communicate, propagate or transport the program for use by or inconnection with the instruction executing system, apparatus or device.The computer-usable or computer-readable medium may for example be, butis not limited to, an electronic, magnetic, optical, electromagnetic,infrared or semiconductor system, apparatus, device or medium ofpropagation, such as for example the Internet. The computer-usable orcomputer-readable medium could even for example be paper or anothersuitable medium on which the program is printed, since the program couldbe electronically captured, for example by optically scanning the paperor other suitable medium, and then compiled, interpreted or otherwiseprocessed in a suitable manner. The computer program product and anysoftware and/or hardware described here form the various means forperforming the functions of the invention in the example embodiment(s).The computer and/or data processing device can in particular constitutea guidance information device which includes means for outputtingguidance information. The guidance information can be outputted, forexample to a user, visually by a visual indicating means (for example, amonitor and/or a lamp) and/or acoustically by an acoustic indicatingmeans (for example, a loudspeaker and/or a digital speech output device)and/or tactilely by a tactile indicating means (for example, a vibratingelement or vibration element incorporated in an instrument).

In accordance with another aspect of the invention, it relates to amedical tracking marker for localizing or tracking an object by means ofa medical tracking system, said marker comprising a first part which isto be fixed on the object, and a second part which is to be connected tothe first part, wherein the first and second parts compriseinterconnecting elements which are adapted to provide a positionallydetermined connection between the first part and the second part, andwherein one of the first and second parts or both parts are trackable bythe tracking system, in particular provided with a reflective surfacewhich can be detected by the tracking system. In one embodiment, thefirst part may be trackable, so that it can—standing alone—be used as atracking marker. In this aspect of the invention, all of the embodimentsdescribed herein and in the claims may be implemented, in particularthose dealing with the adaptations for use with a surgical drape andthose dealing with the various ways of interconnecting the two parts.This inventive aspect, of course, further relates to and comprises amethod as discussed herein, adapted for use with the marker according tosaid aspect.

The invention will now be described in more detail by referring toparticular embodiments and to the attached drawings. It is to be notedthat each of the features of the present invention as referred to heremay be implemented separately or in any expedient combination. Thedrawings show:

FIGS. 1 and 2 an overall arrangement of tracking markers in accordancewith the invention, on a sterile draped object;

FIG. 3 a first embodiment of a marker in accordance with the invention;

FIG. 4 a second embodiment of a marker in accordance with the invention;and

FIG. 5 one variant of a magnetic connection technology for a marker inaccordance with the invention.

FIGS. 1 and 2 show the use of markers in accordance with one embodimentof the present invention. The markers bear the reference sign 12 andconsist of a first part 1 and a second part 2. The first part 1 isfixedly attached to an object 10 which could be a part of a patient'sbody. The object 10 is covered with a surgical drape 20 which isaccommodated between the parts 1 and 2 of the marker 12. In FIG. 2, thebody part 10 has been fitted with a number of markers 12 which arelocalized and tracked by a tracking system 30 which is shownschematically and usually consists of a mounting and two cameras. Thecameras provide a stereoscopic image of the markers 12 and can thusdetect their location and the 3D location of the object 10 after aregistration procedure.

One embodiment of a marker in accordance with the invention is shown inFIG. 3. The first marker part 1 comprises a body 3 featuring a circularrecess 4 on its upper side, while the second part 2 comprises a body 5featuring a circular extension 6 on the side facing the first part 1.The recess 4 and the extension 6 on the respective sides of the parts 1and 2 together form the interconnecting elements which serve to fastenthe parts 1 and 2 to each other. The extension 6 and the recess 4 aredimensioned in such a way that a frictional engagement between the twoelements is provided when a surgical drape is placed between the parts 1and 2. With respect to this and the following embodiments, it may benoted that the position of the first part 1 with respect to the secondpart 2 will in any event be known and/or predictable, i.e. predeterminedby their design and the known thickness of the surgical drape beingused.

The embodiment of FIG. 4 does not employ a frictional engagement butrather magnetic forces. The first part 1 is magnetized in such a waythat its magnetic south is on the lower side 4A and its magnetic northis on the upper or connecting side 4B which comprises the recess. Thesecond part 2, by contrast, exhibits its magnetic south on theconnecting side 6B and its magnetic north on the opposite side 6A whichbecomes the upper or outside (reflecting) side of the marker during use.In this embodiment, the recess on the side 4B and the extension on theside 6B mainly serve the purpose of alignment and can be dimensioned insuch a way that an alignment is guaranteed when a surgical drape isplaced between the sides 4B and 68 when the marker parts 1 and 2 areassembled.

Another way of ensuring an alignment of the two marker parts 1 and 2 isshown in FIG. 5, in which for the sake of clarity only the upper side ofthe first part 1 is shown on the left. On the upper side shown in FIG.5, the first part 1 comprises a circular domain 4C which exhibits amagnetic south and forms an inner circle. At a distance from the circle4C, an outer ring 4D is provided which exhibits a magnetic northpolarity, i.e. there are two polarities on the connecting face, namelythe circle 4C having a magnetic south polarity and the outer ring 4Chaving a magnetic north polarity. The second part 2 is designedconversely, with an outer ring 6D on the connecting side having amagnetic south polarity and an inner circle 6C having a magnetic northpolarity. Bringing together the respective connecting sides of the twoparts 1 and 2 thus not only establishes a connection between the twoparts through a drape via a magnetic force, but additionally ensures areliable alignment of the parts 1 and 2 owing to the different magneticdomains being respectively brought together (4C+6C; 4D+6D) on theconnecting surfaces.

Markers such as are shown in the figures may be used in embodiments ofthe present invention, for example as described in the following:

In accordance with the present invention, a registration device to beused with a medical navigation system may consist of at least three(preferably more, for redundancy purposes) identical marker units 12,each consisting of two parts 1 and 2. The first part 1 of each markerunit is for example fixed to a patient's head, in one embodiment usingadhesive tape. The marker units 12 should be well distributed on theobject to be registered or referenced, for example the patient's head,in order to form a reference geometry. The first part 1 is formed insuch a way that the second part 2 can be fixed on the first part 1, witha drape located between said two parts, wherein they may be fixed interalia mechanically, magnetically or electrostatically, as shown in thefigures and defined above, or by any combination of differenttechniques. At least the second part 2 of each marker 12 has areflective surface (or an active light emitter such as an LED) which canbe located by a tracking system 30, such that the markers and inparticular the reflective surfaces of the second parts 2 serve aspatient references.

Registration can be performed by an independent procedure (for examplesurface matching) or by using the markers. This can be achieved in thesame way as with classic fiducials (pointer registration) or by usingthe positions of the markers determined by the tracking system in orderto perform surface matching.

Another option is to enable the first part 1 to be tracked (for exampleby making it reflective). This would allow independent registration inthe non-sterile, non-draped state, and the software could then adapt theregistration after draping, when the second parts 2 have been mounted,since the position of the second part 2 with respect to the first part 1is well defined.

The first part 1 of each marker 12 can be located after registration,for example using a pointer such as a classic fiducial, even though thepart 1 is not contained in the object scan. Registration would then beperformed by other means (for example surface matching, landmarks,automatically (iCC, REGS, etc.)).

On the other hand, the first part 1 can also be designed as a classicalfiducial which is already mounted for the scan. The first part 1 wouldthen for example comprise a pivot point for pointer registration beforedraping. The second part 2 can also comprise a registration element(pivot point for a pointer) for registering after draping.

Another way of providing a mechanical lock and alignment between the twoparts of each marker would be a conical male/female connection or a morecomplex lock such as a bayonet catch. It may be conceivable inaccordance with the present invention to choose a mechanical connectionor lock which features drape penetration but is well-covered, forexample a small pin, a screw connection or another known oraforementioned type of connection. The parts of the marker may bedesigned to cover the penetration, such that sterility is maintained.Any combination of these and/or the aforementioned locking or connectingtechniques could of course be used within the framework of the presentinvention.

When performing the method in accordance with the invention, analgorithm is used by the navigation system connected to the trackingsystem which locates and tracks the markers. For tracking purposes, thealgorithm can determine the position of the second part 2 relative tothe first part 1 and can take the drape thickness into account. It isalso possible to transfer registration from a first-part 1 configurationto a second-part 2 configuration, compensating for deviations determinedin the two configurations which occur due to inaccuracies in theconnection between the first part 1 and the second part 2, shifts in theskin, camera movement, etc., for example by ignoring the shiftedmarkers. The references tracked may also be only a subset of theavailable markers, in order to allow better set-up flexibility. In thesecases, it is expedient to use more than three markers, i.e. to have acouple of redundant markers.

While it is preferable to use non-invasive means for fixing the firstpart 1 to the object/patient (adhesives, adhesive tape, glue, anadhesive tape already mounted to the first part 1, etc.), it isgenerally possible within the framework of the present invention to fixthe first part 1 to the object via one or more screws or pins(invasively fixing the first part 1).

The second part 2 may be provided with a special identification, such asa specific shape or color or blinking sequence in order for the systemto be able to determine whether it is the first part 1 or the secondpart 2 which is currently being tracked or detected, such that thenavigation system can adapt the referencing algorithm (for example bytaking into account the thickness of the drape and the height of thesecond part 2).

The invention claimed is:
 1. A medical tracking marker for localizing ortracking an object by means of a medical tracking system, said markercomprising: a first part fixable to the object; and a second partfixably connectable to the first part, wherein the connection isconfigured to hold the second part in a predetermined position relativeto the first part and the first and second parts compriseinterconnecting elements adapted to provide a positionally determinableconnection between the first part and the second part while the firstpart is located under a surgical drape and the second part is locatedabove said surgical drape, with the drape placed between the first andsecond parts, wherein the first and second parts are provided with twodifferent magnetic or static domains on each surface involved in theconnection, thereby securing a predetermined alignment of the first andsecond parts which are connected without contacting each other, andwherein the connection is established using forces comprising at leastone of magnetic or electrostatic forces.
 2. The medical tracking markeraccording to claim 1, wherein when the first part is connected to thesecond part the first and second parts are not in direct contact witheach other.
 3. The medical tracking marker according to claim 2, whereinwhen connected the interconnecting elements are not in direct contactwith each other.
 4. The medical tracking marker according to claim 1,wherein when the interconnecting elements are connected saidinterconnecting elements are spaced apart by at least a distancecorresponding to the thickness of a surgical drape, and the connectionbridges said distance.
 5. The medical tracking marker according to claim1, wherein when the interconnecting elements are connected theinterconnecting elements do not penetrate the drape.
 6. The medicaltracking marker according to claim 1, wherein when the first and secondparts are connected, the first and second parts clamp the drape withoutpenetrating the drape.
 7. The medical tracking marker according to claim6, wherein the interconnecting elements clamp the drape withoutpenetrating the drape.
 8. The medical tracking marker according to claim1, wherein the first and second parts are connected by a positiveconnection between the interconnecting elements, and the drape isaccommodated between said interconnecting elements.
 9. The medicaltracking marker according to claim 8, wherein the interconnectingelements comprise at least one of a bayonet catch or quarter-turnfastener.
 10. The medical tracking marker according to claim 1, whereinthe first and second parts are connected by a frictional engagementbetween the interconnecting elements, and the drape is accommodatedbetween said interconnecting elements.
 11. The medical tracking markeraccording to claim 1, wherein the first part of said marker isnon-invasively fixable on the object.
 12. The medical tracking markeraccording to claim 11, wherein the first part comprises an adhesive forfixing the first part to the object.
 13. The medical tracking markeraccording to claim 1, wherein at least one of the first and second partsis provided with a reflective surface detectable by the tracking system.14. A method of referencing an object using a medical navigation system,wherein a medical tracking marker in accordance with claim 1 islocalized or tracked by means of a medical tracking system, and whereinthe object is referenced by means of the first part of the marker in apreliminary referencing step, which is later adapted to form a finalreferencing step for navigation using the second part which has beenfixed to the first part in a positionally defined, pre-determinedmanner, wherein the connection is configured to hold the second part inthe predetermined position relative to the first part without userintervention.
 15. The method according to claim 14, wherein the adaptingstep compensates for and takes into account a shift in the position ofthe marker that occurs when the second part is placed on the first part.16. The method of claim 15, wherein the shift in position is taken intoaccount by ignoring markers which are found to have shifted during themedical navigation.
 17. The method of claim 15, wherein compensatingincludes taking into account at least one of a thickness of the drape; aspecific shape of the second part; characteristics of or inaccuracies inthe system connecting the parts; a shift in the skin; or a movement ofthe tracking system.
 18. A non-transitory computer readable mediumcomprising computer executable instructions adapted to perform a methodin accordance with claim 14.